A persistent problem associated with screen printing is to provide sufficient drying time using ambient art prior to stacking to avoid smearing of newly printed stock. Immediate stacking is undesirable for several reasons. A slight shifting of the stock during the stacking process or when the stock is moved for storage only results in smearing the ink. Premature stacking before drying is complete, results in the sheets sticking together. Excessive drying time is unacceptable for most smaller operations having limited plant space since stock must be removed from the premises relatively soon after printing to make room for additional stock.
Some types of existing screen printing presses have apparatus for the automatic takeoff of printed work, and generally provide for the work to be conveyed to a dryer after removal from the bed or dried by ambient air while on the conveyor. In these types of presses, a transfer carriage having gripper means is reciprocated along a pair of parallel channels. The carriage is first registered to a position adjacent the bed of the press to grip the edge of a piece of stock after it is printed, and then the carriage is placed in a delivery position where the work is released by the gripper means onto the conveyor. Where ambient air is used, the drying time is limited to the length of the convey or travel.
Accordingly, various types of drying schemes have been developed to increase the drying time of newly printed work prior to stacking. One of the most inexpensive and efficient types of dryers yet developed is the wicket dryer. A wicket dryer comprises a plurality of rows of wickets, each row supporting a separate piece of stock, which are mounted at spaced intervals along the upper and lower surfaces of a dryer frame for travel therealong.
The primary advantage of a wicket dryer, in addition to the low costs of purchase and operation, is the excellent and efficient use of plant space. Many existing dryers employ a conveyor belt which advances stock placed thereon through a drying chamber wherein warm air is circulated or heating elements are provided to dry the ink. The problem with these dryers is that stock must be placed face up on the belt and, depending upon the size of the stock to be dried, only a relatively few separate pieces of stock may be placed on the belt at one time unless an overly lengthy belt is used. The artificial dryer also requires large amounts of energy which adds to the cost of printing.
The wicket dryer, however, carries the stock upright as it dries with ambient air, greatly increasing the number of pieces of stock that can be dried at one time in a given space as compared to belt dryers. One problem with the use of wicket dryers prior to the present invention, was that they had to be hand fed since known automatic takeoff apparatus, of the kind described above, were not adaptable for automatic feed to wicket dryers. When a row of wickets reaches either end of the dryer frame, it is pivoted to the opposite surface of the frame. The row of wickets shown in FIG. 1 is depicted in a position mid-way between the upper and lower surfaces of the frame. It is apparent that existing types of takeoff apparatus having a transfer carriage reciprocated along two spaced parallel channels would not be suitable for automatic synchronization with a wicket dryer, since at least one of the channels would interfer with the pivotal movement of the rows of wickets. At best, these types of takeoff apparatus allow only manual feeding of the wicket dryer after the work is released at the delivery position.